Influence of Pulse Duration on X-ray Emission during Industrial Ultrafast Laser Processing

Abstract

Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 1014 W/cm2, a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.

Keywords: X-ray emission; dose rates; hot-electron temperature; laser plasma; pulse duration dependence; ultrafast laser processing.

Figure 1

Measured $\dot{H}$(0.07) dose rates divided by the incident average laser power as a function of the pulse duration for a constant irradiance of 3.6 × 10¹⁴ W/cm² (squares) and 1.6 × 10¹³ W/cm² (triangles). The dose rates were measured with an OD-02 dosimeter at a distance of 25 cm (red squares) and 20 cm (blue triangles). The irradiance was kept constant by adapting the pulse energy. The solid lines were calculated with the extended model given in Equation (9) using the values of the respective experimental settings.

Figure 2

Measured $\dot{H}$(0.07) dose rates divided by the average laser power as a function of the pulse duration (a) and as a function of the irradiance (b) for constant average laser powers of 14 W (red squares) and 18.8 W (blue triangles) at a distance of 20 cm from the plasma. The solid lines were calculated with the extended model given in Equation (9) using the values of the respective experimental settings.